1. Field of the Invention
The present invention relates to a higher order mode dispersion compensating fiber, more specifically, to a dispersion compensating fiber which reduces interference between lower and higher order modes (multipass interference: MPI) and diminishes wavelength dependence of the propagation loss in higher order modes. In addition, this invention relates to a mode converter for a higher order fiber for converting between a fundamental and a higher order mode in a wider band with low loss, and can be applied to a higher order mode dispersion compensating fiber module, etc.
2. Description of Related Art
The rapid increase in demand for data communications has created the need for an optical fiber transmission system which can transmit a larger volume of data at a higher rate.
To accommodate such a need, a reduction in residual dispersion in an optical fiber transmission path has been required and dispersion compensating fibers have been employed.
In addition, since the number of wavelengths multiplexed on a single fiber has greatly increased, the power of optical signals propagating through the fiber has also increased, and prevention of degradation of transmission properties of the fiber due to a nonlinear effect is required. Therefore, an optical fiber having a large effective core area Aeff is desired for configuring an optical transmission path, and thus a dispersion compensating fiber with a large effective core area is desired.
Unlike the fundamental mode (the LP01 mode) used for conventional dispersion compensating fibers, U.S. Pat. No. 5,802,234 discloses a dispersion compensating fiber employing a higher order mode, i.e., the LP02 mode, which is higher than the LP01 mode.
Such a higher order mode dispersion compensating fiber (HOM-DCF) has a substantially large effective core area due to its design, and has a large chromatic dispersion index (i.e., the absolute value of the chromatic dispersion per unit length). For example, a chromatic dispersion index of, for example, −200 ps/nm/km, has been attained.
In such a higher order mode dispersion compensating fiber, interference between lower and higher order modes propagating therethrough cannot be eliminated. Therefore, a mode converter is needed at a junction between a single mode optical fiber and a higher order mode dispersion compensating fiber in an optical transmission path in order to convert the lower order mode to the higher order mode so that propagation of the lower order mode through the dispersion compensating fiber is minimized.
As a mode converter, fiber couplers (Lin et al., “Design Theory and Experiments of LP01-LP11 Mode Converter utilizing fused tapered Fiber Coupler”, Bulletin of the Institute of Electronics, Information and Communication Engineers C-I, Vol. J82-C-I, pp. 587-595, 1999), holey fibers (HF) (S. Choi et al., “A New Type of Mode Converter for Higher Order Mode Dispersion Compensation Based on The Tapered Hollow Optical Fiber”, Proc. Conf. on Lasers and Electro-Optics, CTuAA2, 2001) and long period gratings (LPG) (S. Ramachandran et al. “Bandwidth control of long-period grating-based mode converters in few-mode fibers”, Optics Letters, Vol. 27, No. 9, pp. 698-700, 2002) are used, and multipass interference (MPI) which is defined as the difference in insertion loss between lower and higher order modes can be suppressed to the order of −40 dB.
However, such a level of multipass interference is not sufficient for a practical application, and as long as a mode converter is employed, lowering multipass interference is difficult.
A higher order mode dispersion compensating fiber exhibits a large dispersion, has a large Aeff, and is used for long-haul transmission (e.g., S. Ramachandran et al., “1700 km transmission at 40 Gb/s with 100 km amplifier-spacing enable by high-order-mode dispersion-compensation”, Proc. European Conf. Opt Comm., WeF-2.2, 2001).
One of the important technologies to achieve a practical application of higher order mode dispersion compensating fibers is a mode converter for converting from a fundamental mode of a single mode fiber (SMF) to a higher order mode of an HOM-DCF. As an example of such a mode converter, a converter employing a long period fiber grating and a holey fiber has been reported.
Furthermore, an LP01-LP11 mode converter which employs a fusion tapered fiber coupler using a combination of dual mode (LP01 and LP11) and single mode (LP01) fibers has been proposed.
However, such LPG converters are difficult to manufacture, and their conversion characteristic is susceptible to various environmental factors.
Furthermore, the conversion ratio of HF-type converters from the LP01 mode to the LP02 mode (hereinafter referred to as “LP01-LP02 conversion ratio”) is low.
Furthermore, Lin does not describe any converters capable of converting to the higher order mode LP02.
Accordingly, the present invention is directed to a higher order mode dispersion compensating fiber which reduces propagation in modes lower than a higher order mode, diminishes interference between lower and higher order modes, and further decreases multipass interference.
In addition to solving the first problem mentioned above, the present invention is directed to a higher order mode dispersion compensating fiber which diminishes wavelength dependence of the propagation loss at higher order modes.
The present invention is further directed to a mode converter for a higher order fiber which converts between a fundamental mode and a higher order mode in a wider band with low loss.